Browning
|
DIN 50938 |
|---|---|
| Area | Coating of components |
| title | Burnishing of components made of ferrous materials - Requirements and test methods |
| Latest edition | 2018-01 |
| ISO | 11408 |
Browning (from French brunir , browning) is used to form a weak protective layer (patina) on mostly iron-containing surfaces in order to reduce corrosion . When the workpieces are immersed in acidic or alkaline solutions (e.g. sodium hydroxide solution ) or molten salt , black mixed oxide layers of iron (II, III) oxide ( Fe 2 O 3 and Fe 3 O 4 ) are formed. The browning is not a coating .
properties
Due to the small thickness of the conversion layer of around 1 µm, the burnished workpieces are largely dimensionally stable. Because of the porosity of the burnishing layer, the corrosion protection is only slight; however, it can be significantly improved by oiling or greasing. The layers are largely resistant to bending and abrasion and are temperature resistant up to around 300 ° C. A browning also acts as a primer for surface coatings such as B. Paints.
The area of application is in machine and tool construction . A special application is the bluing of handguns .
The procedure is standardized in Germany in DIN 50938. A distinction is also made there between one-, two- and three-bath burnishing.
In addition to protection, the browning is also often used to visually enhance objects, for example to create an "antique" look, such as furniture fittings and screws made of brass . For this, browning can be done with vinegar essence ( acetic acid ).
Blackening
A method used in iron cookware and forged products to reduce the susceptibility to corrosion is the burning off of linseed oil on the iron surface, which is also known as burning in or black burning . To do this, the metal is heated between glowing coals and then quenched with linseed oil. Alternatively, the oil can be applied first and the object then heated. It usually takes several passes to get a good result. The temperature should be around 400 ° C to 700 ° C (dark red). The oil then smokes visibly when it is applied.
Similar processes using linseed oil are the varnish firing of copper and copper alloys and the patination of aluminum at temperatures of up to 400 ° C.
Some methods of forming oxide layers by immersion in molten salts are also known as blackening.
However, these baking processes should not be confused with the baking of printing inks and varnishes at temperatures below 200 ° C.
literature
- Hansgeorg Hofmann, Jürgen Spindler: Surface technology processes. Basics - pretreatment - coating - surface reactions - testing. Fachbuchverlag Leipzig in Carl Hanser Verlag, Munich et al. 2004, ISBN 3-446-22228-6 .